Hydrogen Production Cost from Fermentation Originator: Katie Randolph and Sarah Studer (DOE)
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DOE Hydrogen and Fuel Cells Program Record Record: 16016 Date: February 14, 2017 Title: Hydrogen Production Cost f rom Fermentation Originator: Katie Randolph and Sarah Studer (DOE) Reviewed by: Zhiyong "Jason" Ren, Hong Liu, Alexander Beliaev, and Jamie Holladay Approved by: Sunita Satyapal Date: February 27, 2017 (DOE) Item The projected cost to produce hydrogen (H2) from dark fermentation of biomass (corn stover) using techniques and strains currently in development at the laboratory scale is greater than $50/kg1 (untaxed, high system production rates). However, it is expected to drop dramatically in the future to $5.65/kg by 2025, if assumed improvements in the technology and high volumes are realized. Two cases were considered, a projected Current year case based on 2015 technology using performance and design parameters that have been simultaneously demonstrated in the lab at low reactor volumes, and a projected Future case based on projected technological advancements by 2025. The cost analysis was performed using the Hydrogen Analysis version 3.101 (H2A Production v3.101) model and its associated 2 3 assumptions for a centralized production facility with a production capacity of 50,000 kg H2/day. The analysis utilizes a system design based on lab-demonstrated hydrogen production procedures4 and using capital costs derived from a 2013 NREL report5 on the production of hydrocarbons from lignocellulosic compounds. Summary The modeled costs (untaxed, delivery and dispensing not included) to produce hydrogen are summarized in Table 1 for the two cases studied. The baseline costs are the projected costs to produce hydrogen for the projected Current and projected Future cases. The low and high values are included to reflect a range of uncertainty (±25%) in installed capital costs (with all other techno-economic inputs the same as in the baseline cases). Table 1: High-volume cost projections for hydrogen production from a centralized facility with 1 50,000 kg H2 /day production capacity using 2014 (projected Current) and 2025 (projected Future) technologies. Optimistic Value Baseline Conservative Value Case Study (2007$/kg H2) (2007$/kg H2) (2007$/kg H2) Current Case (2015) $59.76 $67.71 $75.67 Current Case (2015) with $40.88 $51.02 $61.16 byproduct credit Future Case (2025) $7.68 $8.56 $9.43 Future Case6(2025) with $3.40 $5.65 $7.91 byproduct credit Analytical Basis Analyses of the cost to produce hydrogen at a central production facility with a plant capacity of 50,000 kg 7 H2/day were performed using the H2A Production v3.1 model. Two technology years were considered, projected Current8 (2015) and projected Future (2025).9 The forecourt production capacity H2A model was not considered in this analysis. 1 There are no commercial dark fermentation hydrogen production facilities on which to base the system designs. Consequently, relevant techno-economic analysis inputs were derived for a hypothesized system. The process design for this projected hydrogen fermentation plant draws from two main sources: a hydrogen production fermentation plant previously conceptualized in 2009,10 and a design and cost report for the production of lignocellulosic ethanol.5 A 2013 National Renewable Energy Laboratory (NREL) report was supported and supplemented by data from previous versions of the report.11,12 Data from these reports were adjusted to reflect recent technological progress and thinking (See Figure 1) and to adapt for hydrogen production. The alterations primarily consist of elimination of the distillation columns and scaling of the waste water system. The distillation columns are not required in the hypothesized system design as the system is not producing and purifying ethanol. The waste water treatment was scaled according to the size of the system and the content of the organic components in the waste stream. These organic compounds were modeled as being converted to biogas in the waste water treatment center. Consistent with the 2009 analysis and the laboratory data, hydrolysis pretreatment of cellulose and hemicellulose have been combined into one reactor, with combined saccharification and fermentation assumed to occur in a subsequent single reactor. In accordance with the 2009 analysis, Clostridium thermocellum converts cellulose to hydrogen and other byproducts in the projected Current case analysis. Clostridium thermocellum can also be combined with other microbes to create a microbial consortium that is capable of converting both cellulose and hemicellulose derivatives to hydrogen. This microbe consortium is modeled for the projected Future case as a technological improvement that will increase corn stover to hydrogen yield. Reaction parameters such as reaction rates, compound concentration, and product yields were provided by NREL.13 Capital equipment design, cost, and performance data were gathered from the literature,5 modified as appropriate to meet the hydrogen fermentation plant needs, and used to populate a set of baseline cases. Inputs to the H2A model fell into five primary categories: 1. Engineering system definition, 2. Capital costs, 3. Operating costs, 4. Variable and fixed expenses, and 5. Replacement costs. 2 Figure 1: Process flow diagram used as the model system to project the cost to produce hydrogen via fermentation at a central production facility with a 50,000 kg/day capacity. For each technology year considered, a model was created to determine the capital cost based on production volume, molar conversion of sugars to hydrogen, heat and energy requirements, and energy byproducts. Both projected Current and projected Future cases envision a hydrogen fermentation plant (Figure 1) in which feed material (modeled as corn stover, with cost of preparation for processing included) is delivered to the plant. Feedstock is first broken down via the pretreatment process.14 The partially converted feedstock is then sent to a fermentation reactor in which two main reactions occur: (1) cellulose is hydrolyzed to hexose sugars, and (2) sugars are fermented into hydrogen and other products. For the projected Current case, C. thermocellum resides within the fermentation reactor and ferments only hexose sugars. For the projected Future case, a consortium of microbes, based on C. thermocellum, resides in the fermentation reactor and ferments both hexose and pentose sugars. The fermentation reactors are modeled as operating at 55°C for a given batch time. The fermentation batch time was determined through a cost optimization study, by plotting projected hydrogen cost as a function of fermentation time (see Figure 2). Maximum fermentation time was limited to 74 hours, as NREL data showed maximum conversion at that limit. The optimization curves, based on 2015 lab results, suggest the minimum cost of hydrogen corresponds to a fermentation time of 74 hours for both the projected Current and projected Future cases.15 3 A B Figure 2: Optimization Curves for Fermentation Cases A) projected Current case and B) projected Future case based on the modeled corn stover loadings and other variables. The H2 and CO2 gaseous products are vented from the fermentation reactor and separated from one another via Pressure Swing Adsorption (PSA). After fermentation, the broth is filtered, with the solids fraction (mostly lignin) used for energy recovery, and the liquid fraction (a dilute mixture of ethanol, acetate, and other organic acids) sent to waste water treatment. The waste water treatment plant is based on anaerobic digestion to create a byproduct gas of mostly methane. The byproduct gases, as well as the lignin, are combusted to generate thermal energy to heat the system.16 The excess thermal energy is converted to electricity in a gas turbine electrical generator and sold to the grid for byproduct credit equivalent to $11.93 and $8.19 per kg of hydrogen produced for the projected Current and projected Future cases, respectively.17 In the cases without byproduct credit, the hydrogen cost is higher since there is no revenue collected from electricity sales, but the cost increase is partially offset by a reduction in system capital cost as there is no need for the gas turbine. Fuel cell conversion of the byproduct gases into 4 electricity was considered but ultimately rejected due to a desire to focus the cases on fermentation technology and to leave them unencumbered by uncertainty of fuel cell capital cost projections. Key differences between the projected Current and projected Future case are: 1) A change in fermentation broth concentration18 from 12.8 g/L (projected Current) to 175 g/L (projected Future).19,20 2) A change from C. thermocellum (capable of converting only hexose sugars) in the projected Current case to a microbial consortium (capable of converting both hexose and pentose sugars) in the projected Future case. 3) An increase in peak molar conversion of sugars to H2 from 1.16 mol H2/mol sugar (projected 21 Current) to 3.2 mol H2/mol sugar (projected Future). The yields were determined based on experimental data from NREL. NREL ran fermentation studies with acid hydrolysis pre-treated corn stover (PCS) and Avicel22 feed stocks, creating fermentation broth with cellulose concentrations of 1 g, 2.5 g, and 5 g cellulose/L.23 The peak molar yields ranged from approximately equivalent to 1.16 mol H2/mol hexose to approximately 3.2 mol H2/mol hexose, at 74 21 hours. The selected operating points for lowest system cost are 1.16 mol H2/mol hexose at 74 hours for a 24 12.8 gram corn stover/L broth concentration for the projected Current case, and, for the projected Future Case, 3.2 mol H2/mol sugar (pentose and hexose) at 74 hours for a 175 g corn stover/L broth concentration. The projected Future case model used the highest yield demonstrated by NREL. Note, however, that this yield occurred at the lowest fermentation broth concentration tested (1 g cellulose/L) while the projected Future case is based on achievement of this high molar yield at the projected Future broth concentration of 175 g/L.